It really depends on theIt really depends on the flower but it's usually fine to transplant annuals. For petunias, you want the plants spaced about 1 foot apart. See our petunia page: http://www.almanac.com/plant/petunias
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http://www.almanac.com/plants/type/Flower
When you transplant, do it on a cloudy day and try not to disrupt the roots or soil and transplant at the same depth they were growing in the containers. Water thoroughly after transplanting to avoid excessive wilting.